I have shared versions of this on other platforms before, so I might as well make a tumblr edition: here some tips for MtG portfolios I gathered and might be interesting for some people who follow me.
1. Since this is a trading card game, here comes the obvious one first: Always keep in mind that these are card illustrations, they have to be readable in super small. Which means that strong silhouettes and value structures are a must have. If you work digital, check the zoomed out version on regular basis, or even have some jpgs to check their thumbnails in your file browser. That can give you an idea about their readability. Traditionally you can of course take some steps back, or take some photographs to look at smaller previews on your devices.
Also: print illustrations often come out darker than their screen versions, be careful with your darks! It's rather easy for things to go muddy, even if they look good on screen. In doubt, increase the brightness a bit. It's okay to have different versions for screen and print to meet their needs.
2. Be versatile about your topics and compositions. Zoom in, zoom out. Don't fall into the trap of your own comfort zone zoom level of showing things, or one way of doing things.
It can be positive to offer purposefully unusual options.
3. Be aware of the focus. If you have a magician with a staff, ask yourself if the card is about the staff(artifact), the mage (creature) or perhaps even the spell. The composition and focus of the illustration should shift accordingly! Clear action is important for readability – since that is not just visual hierarchy here, but also storytelling. Which brings me to the next point:
4. Good narrative matters, but mechanics matter even more. So, again, be very aware of your illustration's focus. You can potentially add extra elements for the story to make it more fun, but it should not get too convoluted, and even less should it distract from what the card it actually about.
If you come up with your very own ideas for a portfolio this is of course much more open than if you work from a description. But you can find a bunch of official MtG descriptions online which are super useful for training.
5. Show care. Plan the illustration, get the references in place. It's the best time to get good habits in place, and really finish the pieces. Don't make them weaker by going too fast, that is not convincing. It just lets people assume worse things for tight deadlines.
This does not mean everything needs to be rendered to death - but shape design should remain thoughtful and purposeful even where soft and lost edges are used.
6. It's potentially okay to have your specific stylistic or thematic niche. It can mean less assignments at times, but can also mean more special ones. It's cool though for your voice to be visible as long as the other needs of the product are met.
7. Never stop using those references. Get them, make them, use them - take them seriously. (at least for any of the more realistic styles). It's one of the most repeated tips for any student to actually just use more references. They do a ton to get complicated things like anatomy and lighting right, but also cultural references and versatility. Many of the best Magic artists also make the best references – it's not a coincidence. Learn from the people who have already established themselves, they have great wisdom to share.
8. Your quality has to match the current roster.
Yeah, sorry, no way around that one. You need at least to be as good as the currently "worst" artist in the roster to have a chance. And the ADs need to be sure that even on a bad day your art can meet their quality bar. Which is the reason why you likely need several art pieces at the required level, to prove it wasn't just some lucky fluke.
Though once you're really there, that also means a bit less pressure to perform, since you're likely comfortable at your skill level and can only go up from there.